For a Visitor Location Register (VLR) to do an attach/detach audit of mobile devices and corresponding VLR space management, the VLR normally uses some timers or timestamps for each subscriber/user equipment (UE) currently registered in the VLR. A subscriber or UE's activity timestamp is updated in the VLR when the subscriber/UE has an activity such as location update, a call, a Short Message Service (SMS), etc. When a subscriber/UE does not have any activity, or does not report any activity for a long time, its activity timestamp in the VLR will become old. That is, the subscriber/UE ages when no activity occurs or is reported to the VLR, and this process is referred to as “aging”.
With current wireless technology development, more and more special usage UEs such as Internet of Thing (IoT) UEs are connected to a Mobile Switch Center (MSC)/VLR through an SGs interface. In 3GPP, the SGs interface is an interface between the VLR and a Mobility Management Entity (MME).
Some of these special usage UEs may only use MSC for services such as SMS, and use packet core for other services. Some of these special usage UEs may have very infrequent contact with the VLR/MSC (either through the SGs interface or through a 2G/3G radio interface). For example, these special usage UEs may only do a periodic tracking area update (PTAU) with the MME over a long period of time, such as, an in-vehicle UE in a car parked in an airport parking lot. Based on 3GPP specifications, the MME normally does not notify the VLR when it receives a PTAU. Thus, if an SGs-associated UE only does PTAUs and there are no other VLR/MSC core activities for a long time, its “age” in the VLR will become old. On the other hand, due to VLR space/memory management, VLR may need to delete the older/aging subscribers/UEs to make room for new incoming subscribers/UEs to the VLR.
One problem with existing solutions is that per current 3GPP specifications, e.g. TS 29.118, when a UE is attached for Evolved Packet System (EPS) and non-EPS services, a serving VLR shall stop the implicit detach timer so that a UE is not marked as detached in the serving VLR even if the UE only does PTAU with the MME and there are no activities with its serving MSC for a long time. However, this existing 3GPP solution does not align with the VLR space management when the VLR needs to delete older or aging inactive subscribers/UEs, including the attached ones, to make room for new incoming subscribers/UEs. Also, deletion of an SGs-associated subscriber/UE such as an inactive or stationary aging attached in-vehicle UE would cause such UE to not function properly with the MSC/VLR.
Another problem with the existing solutions is that current 3GPP specifications do not specify any different treatment for SMS-only UEs in terms of Temporary Mobility Subscriber Identity (TMSI) allocation/reallocation. For example, when a serving VLR processes a SGsAP-Location-Update-Request message, the VLR may allocate a new TMSI to the SMS-only UE unnecessarily.